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Ascariasis (Ascaris lumbricoides)
Etiology
Ascariasis is caused by the nematode, or roundworm , Ascaris lumbricoides. Adult worms of A. lumbricoides inhabit the lumen of the small intestine. The reproductive potential of Ascaris is prodigious; a gravid female worm produces 200,000 eggs per day. The fertile ova are oval in shape with a thick, mammillated covering measuring 45-70 µm in length and 35-50 µm in breadth ( Fig. 317.1 ). After passage in the feces, the eggs embryonate and become infective in 5-10 days under favorable environmental conditions. Adult worms can live for 12-18 mo ( Fig. 317.2 ).
Epidemiology
Ascariasis occurs globally and is the most prevalent human helminthiasis in the world. It is most common in tropical areas (South America, Africa, Asia) where environmental conditions are optimal for maturation of ova in the soil. Approximately 1 billion persons are estimated to be infected. Although the number of cases in the United States is not known precisely, the highest prevalence is thought to be in high-poverty areas of the South and Appalachia. Pig farming in Maine is also associated with Ascaris species. Key factors linked with a higher prevalence of infection include poor socioeconomic conditions, use of human feces as fertilizer, and geophagia. Even though infection can occur at any age, the highest rate is in preschool or early school-age children. Transmission is primarily hand to mouth but may also involve ingestion of contaminated raw fruits and vegetables. Transmission is enhanced by the high output of eggs by fecund female worms and resistance of ova to the outside environment. Ascaris eggs can remain viable at 5-10°C (41-50°F) for as long as 2 yr.
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